This invention relates to an ultrasonic testing method for detection of flaws in bails, and more particularly to an ultrasonic testing method for balls in which the accuracy of flaw detection is enhanced by specifying the test frequency of a probe, the radius of curvature of a tip portion of the probe and the diameter of an oscillator of the probe.
As a testing method for detection of flaws in steel materials, steel plates, forgings, etc., there has hitherto been used an immersion type ultrasonic testing method.
The conventional immersion type ultrasonic testing is generally used where the material to be tested is comparatively large and the flaws to be detected are as large as several millimeters or above. Therefore, the testing system used for the conventional ultrasonic testing is not a special one. Also, the probe used for transmission and reception of an ultrasonic wave during testing is an ordinary one, namely, an immersion type probe which has a simple planar oscillator with a large diameter.
For products to be used under such severe conditions that even minute flaws, if any, would cause problems in practical use of the products, a system for ultrasonic detection of even minute flaws ranging down to several hundreds of micrometers in size has been adopted, in order to enhance reliability of flaw detection.
In an ultrasonic testing method for detecting minute flaws, a focus-type probe in which a concave resin lens is attached to the above-mentioned planar oscillator or in which the oscillator itself is shaped to be concave has been used.
In addition, research has been made in recent years on the use of ceramics for bearing members and the like, which are particularly required to have high reliability.
Because such ceramics are brittle materials, a testing method with high resolution has been desired for detection of flaws in ceramic products. In the recent art of ultrasonic testing for detection of flaws in ceramics and the like, therefore, attempts have been made to enhance the sensitivity and accuracy of flaw detection by raising the test frequency from previously used values of about 0.5-10 MHz to higher values of 15-100 MHz or by using a computer to perform image processing, and so on.
In application of the above-mentioned prior art, however, the materials to be tested have been limited to comparatively large products of a simple shape, such as flat plates, circular cylinders, prisms, circular tubes, etc. Also, the flaws detected have been limited to flaws being not smaller than 0.5 mm, at best, and located at a depth of several millimeters or more from the surface of the material under test. Accordingly, attempts to detect flaws in the surface and sub-surface of products having radii of curvature of 10 mm or less, such as a ball bearing, by the prior art have failed because of scattering and reflection of the transmitted ultrasonic wave at the surface of the product and because of complicated refraction of the propagated ultrasonic wave.
An ultrasonic testing method for detection of minute flaws in the surface, and within a depth of 2 mm from the surface, of bearing rolling elements in the form of balls, cylinders or the like has been proposed in Japanese Patent Application Laid-Open (KOKAI) No. 63-243751 (1988) corresponding to USSN 172244. In this method, a focus-type probe and the rolling element as the material to be tested are disposed with a predetermined amount of eccentricity therebetween, and ultrasonic flaw detection is carried out to detect minute flaws in the surface and the sub-surface of the material. The method, however, is not applicable to materials to be tested which have a radius of curvature of not more than 10 mm or which have a special curved surface.